The proposed research involves studies on the development and structure of the leaves of maize and barley, utilizing bright-field, transmission electron, epifluorescence and confocal laser scanning microscopy. The developmental and structural studies will be complemented by experimental studies aimed at determining: (1) the pathways followed by photoassimilates in source and sink leaves; (2) the sites and mechanisms of phloem loading and unloading in source and sink leaves, respectively; (3) the structural modifications associated with the sink-to-source conversion; and (4) the pathways followed by water and solutes enterhing the leaf apoplast in the transpiration stream.